This invention relates to a process for developing electrostatic images formed on an electrostatic image bearing surface, particularly to a process for developing electrostatic images by forming a thin and uniform toner layer of a specific toner which is insulating and substantially nonmagnetic on a toner carrying member and permitting the toner of the toner layer to jump onto the electrostatic image bearing surface, and to a toner therefor.
The developing methods of the prior art in electrophotography, electrostatic recording, etc., may be classified broadly into dry system developing methods and wet system developing methods. The former methods are further classified into methods employing two-component developer and methods employing one-component developer. Those belonging to the two-component developing methods include, depending on the carriers employed for delivering toner, the magnetic brush method employing iron powder, the cascade method employing a bead carrier, and the fur brush method employing fur.
Those belonging to the one-component developing methods include the powder cloud method in which toner particles are used in atomized state; the contact developing method (also called as toner developing method) in which toner particles are permitted to contact directly the electrostatic surface; the jumping developing method, in which toner particles are not permitted to contact directly the electrostatic image, but rather the charged toner particles are permitted to fly through the electrical field possessed by the electrostatic latent images toward said latent image surface; and the magne-dry method in which development is effected by permitting magnetic electroconductive toner to contact the electrostatic latent image surface.
In one-component developing methods, the magne-dry method employing a magnetic toner and the contact developing method employing a non magnetic toner involve a problem that the toner contacts all the surface to be developed, namely, both the image portion and the non-image portion indiscriminately, whereby the toner is readily attached even to the non-image portion to cause contamination as ground fog. With respect to fog contamination, a similar tendency is observed also in the two-component developing method. Also in the powder cloud method, attachment of the toner particles in powdery state onto the non-image portion cannot be avoided, thus having the same problem that the ground fog cannot be removed. The jumping developing method as disclosed in Japanese Laid-Open Patent Applications No. 43027/1979 and No. 18656/1980, comprising disposing an electrostatic image bearing member having an electrostatic image on the surface thereof and a toner carrying member at the developing zone with a clearance therebetween, causing a toner to be carried on the toner carrying member to a thickness less than the distance defined by the above clearance and transferring the toner to the above electrostatic image bearing member at the developing zone, has the advantage not inadvertently producing the fog as mentioned above, and therefore may be appreciated as an electrostatic image developing method providing images which are high in faithfulness and stable in image quality. This method can provide a developing method which is excellent in durability such as successive use characteristic, and stable under various environmental conditions such as high temperature and high humidity or low temperature and low humidity. However, as a problem in this method, the toner layer on the toner carrying member is required to be uniform, otherwise, there is a fear that density irregularity of image or ground fog may be generated to give an unsightly image.
We have investigated in this developing method by use of non-magnetic or weakly magnetic toners and consequently found that it is important to control precisely the quantity of static charge possessed by the toner in order to improve the problem as mentioned above in the jumping developing method.
Further, we have found that the charging characteristic of a toner concerned with the static charge quantity can be controlled by controlling the molecular weight distribution of the resin constituting the toner.